<?php
//Include mysql functions
include("mysql.php");

//Connect to db
connectdb();

//Escape the GET to find the username and various other stuff
$id = mysql_real_escape_string($_GET['id']);

//Let's lookup the username
$username = mysql_fetch_array(mysql_query("SELECT username FROM users WHERE id=\"".$id."\""));
//We set the username as we received from the db
$username = $username['username'];

//We greet the user
echo "Hi ".$username;

?>
